The present invention relates generally to digitizing tablet systems and more particularly to digitizing tablet systems which include a conductive grid structure and a stylus structure which is inductively coupled to the conductive grid structure.
Digitizing tablet systems are well known in the art and are used in a variety of applications. These systems generally contain a tablet including a housing having a two-dimensional work support surface and a grid of conductive elements. The stylus which interacts with the grid structure generally contains one or more electric coils. The coil(s) in the stylus are inductively coupled to the grid in the tablet by energizing either the coil or the grid with an AC voltage signal. The voltage signal induced in the other component is then measured. The measured signals are then used to determine the position of the stylus relative to the grid.
The grid for a tablet system may be configured by having a number of parallel wire loops forming horizontal loops across the grid and another set of parallel wire loops perpendicular to the horizontal loops forming vertical columns in the grid. Each loop is a discrete sensor for sensing the intensity of the current or voltage induced by the coil or coils of the stylus. To determine the location of the stylus when it is between loops, an interpolation must be performed. The basic magnetic equations which relate the field intensity of the induced current to the location of the stylus coils will generally be in the form of a parabola in the region of interest. By comparing the intensity of a current sensed in adjacent loops, one can determine the location of the coils.
The signals received from the loops are demodulated and filtered. Demodulation produces a DC signal proportional to the original sine wave plus AC components which are generally at multiples of the sine wave frequency. To get precision measurements, the resulting signal is generally low-pass filtered to remove the higher-frequency components and also to remove extraneous noise. A typical low-pass filter is not optimum for several reasons. For a given response time, the band-width for noise must be larger than it needs to be since the normal low pass filter must wait for old information to decay before it can work on the next set of information.
The conductive grid of a digitizing tablet is subject to capacitive interference. For example, a radio or even a hand would carry an electric field which would act as a capacitor on the lines of the grid. Thus, it would be desirable to provide a mechanism for shielding the grid from interference.